The invention generally pertains to a process for the preparation of double metal oxide ceramic powders which contain at least one metal of Group IIIA, inclusive of aluminum and gallium, and at least one metal of Group IVB, inclusive of titanium and zirconium, which process involves the formation of a mixed metal precursor that is subsequently calcined, or otherwise converted, into the desired oxide. In particular the invention pertains to the preparation of aluminum titanate. The invention also pertains to a mixed metal precursor useful in the preparation of said double metal oxides.
Double metal oxide powders are known ceramic compounds, which find usage, e.g., as abrasives or as thermal insulators. Aluminum titanate, Al.sub.2 TiO.sub.5, in particular, is a hard abrasive and is known to have a low thermal expansion and hence a high thermal shock resistance.
Processes for the preparation of double metal oxides of the type indicated above which involve the formation of a mixed metal precursor are known in the art.
One such process has been disclosed in the Journal of Materials Science 21 (1986) 4431-4435. It involves controlled hydrolysis of organometallic precursors such as aluminum and titanium alkoxides dissolved in an alcoholic medium. A significant drawback to this so-called sol-gel process is the criticality of the rate of hydrolysis. In order to obtain a product with the desired ratio of both metals the relative hydrolysis rates of both alkoxides need be carefully adjusted by, e.g., varying the nature of the alkoxide substituent and/or varying the reaction temperature. In commercial practice it is desired that the metal ratio in the mixed metal oxide be adjusted in a more simple and straightforward manner. A further drawback to the sol-gel process is the need to use alkoxides. For economical reasons a process is desired in which metal salts are applicable as well, since these are generally cheaper. Still another drawback to double metal precursors of the alkoxide type is their flammability and the heat evolved in the exothermic decomposition reaction, which renders these compounds hazardous in preparing the desired double metal oxides.
A process of the type referred to above in which the use of metal salts is contemplated has been disclosed in U.S. Pat. No. 3,825,653. According to the disclosure of this patent, sinterable aluminum titanate powder is prepared by coprecipitating alkoxide or halide compounds of aluminum and titanium as a hydroxide. The process comprises the steps of forming a solution of an aluminum compound in the +3 valence state and a titanium compound in the +4 valence state, coprecipitating the aluminum and the titanium from the solution as aluminum titanium hydroxide, filtering and drying the precipitate, and thereafter calcining the mixed metal hydroxide precursor to form aluminum titanate powder. In the embodiment where metal alkoxide compounds are used, the solvent should be a hydrocarbon liquid, preferably benzene. In another embodiment water-soluble metal salts are used, which after mixing should be rapidly added to an aqueous solution containing a hydroxide. Both alternative embodiments display significant drawbacks. Besides the above-mentioned disadvantages of using alkoxides, the alkoxide embodiment disclosed in U.S. Pat. No. 3,825,653 displays a further drawback in that organic solvents are required which for environmental and health reasons should be avoided. In this respect as well as with regard to the above-mentioned economical reasons the water-soluble salt embodiment is to be preferred. However, this has proven to be unsuitable for any commercial-scale use, since the critical step of mixing the two metal salt solutions and adding the mixed solution to the aqueous hydroxide solution unavoidably leads to uneven precipitation. Uneven precipitation means the undesirable result that one of the metal compounds precipitates preferentially, or even that a single metal precipitate is formed. Hence, said known process is insufficiently controllable to be feasible in commercial-scale use.
The present invention has for one of its objects to provide a process for the preparation of mixed metal oxides as indicated above that avoids the use of hydrocarbon solvents, even if organometallic starting materials are used. A further object of the present invention is to prevent uneven precipitation, even if mixed aqueous metal salt solutions are used. Still another object of the present invention is to provide a process of the above-identified type in which the metal ratio in the resulting mixed metal oxides can be adjusted simply by adjusting the ratio of the starting compounds. A further object of the invention is to provide a double metal precursor that can be used with advantage in preparing double metal oxides, and which avoids explosion hazards.
These and other objectives are met by the process and product as described in the following summary and detailed description of the invention.